In an access point (AP)-based wireless local area network (WLAN), multiple stations (STAs) may be associated with a given AP at any given time. If the multiple-access scheme is Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) such as in 802.11 WLANs, any STA may transmit a data packet (also called “frame”) to its associated AP at any given time. Typically, the AP determines which of its associated STAs has transmitted a packet after the packet has been completely received and decoded, based on the source address contained in the medium access control (MAC) header of the packet. In order to make this determination, the AP generally needs to have received the whole packet, because the error detection bits covering both the MAC header and the MAC payload are usually received at the end of the packet transmission.
An AP may also be equipped with a smart antenna in order to improve the signal-to-noise ratio (and hence the throughput and/or coverage) of AP-to-STA transmissions as well as STA-to-AP transmissions. The term “smart antenna” in this context may refer to a set of N antennas that have different radiation patterns, such as by pointing in different directions, or a smart antenna may include an omni-directional antenna, which is capable of transmitting beams in a plurality of separate directions.
Ordinarily, the transmitter or receiver of a node (AP or STA) selects the most appropriate antenna, or beam, for communicating with its counterpart. The most appropriate beam is typically the one that results in the highest signal-to-noise-plus-interference ratio (SINR) at the receiving node in the case of dedicated connections, where a node is transmitting a data packet to another specific node.
Additionally, Mesh Points (MPs), which are similar to STAs in a mesh architecture, may also be equipped with smart antennas in order to improve the signal-to-noise ratio of received signals or for other purposes such as interference reduction.
In the case where more than one STA is associated with an AP, the multiple-access scheme in 802.11 may render difficult the selection of the most appropriate beam for the reception of packets at the AP. This is because STAs may be located in any direction relative to the AP. As a result, the most appropriate beam may not be the same for different STAs. Since the identity of the STA is not known before completion of the reception of the packet, the AP cannot use this information to decide which antenna or beam to select for the reception of the packet. The same problem exists for MPs in a mesh architecture when an MP can be linked to more than one other MP.
To address this difficulty, several alternatives may be employed. However, there are drawbacks to each alternative. For example, the AP could restrict itself to the use of an omni-directional beam for all packet receptions, but it would then lose the potential gain from the use of a smart antenna.
Alternatively, the AP could use the signals from multiple beams simultaneously and combine them or select the best among them. The drawback to this solution, however, is that it increases the complexity of the receiver, because the signal from multiple beams must be demodulated.
In another alternative, the AP could, just after the start of packet reception, switch among all its available beams in a successive manner, pick the beam that resulted in the best signal quality, and switch to this beam for the remaining duration of the packet reception. This approach has the drawback that the AP risks incorrectly receiving some bits while it is cycling through the least suitable beams for a particular packet, resulting in the loss of the packet.
Another alternative is that the AP could try decoding the medium access control (MAC) address of the sender (contained in the MAC header of the packet) using an omni-directional antenna, and then use the most appropriate beam for the particular STA identified in this way for the remaining of the packet. The problem with this approach is that the MAC header is transmitted at the same rate as the remaining portion of the packet. If the omni-directional antenna does not offer sufficient gain for adequate signal quality for the MAC payload, it is unlikely that the MAC header would be decoded correctly. In the opposite case, there would be no need for the use of a smart antenna in the first place.
In yet another alternative, the STAs could be constrained to send every packet using Request-to-Send/Clear-to-Send (RTS/CTS) procedure. This would allow the AP to identify the sending STA before the arrival of the data packet. However, this is at the cost of a significant throughput penalty due to the overhead of the RTS and CTS packets, which has the effect of potentially nullifying the purpose of using smart antennas.
The AP could poll STAs using different beams in succession. However, it is inconvenient to attempt to predict the time to spend on each beam in a system with bursty traffic such as that in a wireless LAN, and it is also difficult to prevent STAs from responding to a poll sent using a beam that is sub-optimal, but recognizable, for them. This is due to the necessary overlap between antenna patterns and the irregularities of the radio environment, such as shadowing.
Another alternative could be to add an identifier to the Physical Layer Convergence Protocol (PLCP) header to allow the AP to determine which beam it should use for the reception of the MAC frame. This identifier could correspond to a beam identifier or to a station identifier. Although this solution may have the least overhead, it involves changes in the lower layers of the WLAN protocol, which may not be acceptable in some scenarios.
Another problem associated with some of the solutions above is that they rely on the assumption that if the AP is able to identify from which STA the frame originates, the AP will automatically know what beam it should use when receiving packets from this STA. This may only be true if the AP performed a beam scanning procedure prior to the transmission and reception of frames. In fact, even in the case where an AP has already performed such a beam scanning procedure, the AP might not be able to determine which beam will maximize the reception of packets from the desired STA since each STA can move and the RF environment may vary.
It should be noted that all of the above problems are present in a mesh network when an MP is equipped with smart antennas. Like an AP, an MP can receive packets from a multitude of WLAN nodes, such as neighboring MPs. Thus, in a mesh system using an access mechanism similar to the contention-based mode used in typical 802.11 systems, an MP equipped with smart antennas has no means for knowing which MP will send the next packet prior to the packet being sent. Accordingly, this is an obstacle to the use of the smart antenna capabilities of the MP when receiving packets.
It would therefore be desirable if a method and apparatus existed that overcomes the drawbacks of prior art wireless systems.